The least emphatic separating mark of punctuation, used:

1. Between adjectives which each qualify a noun in the same way, e.g.

A cautious, eloquent man.

But when adjectives qualify the noun in different ways, or when one adjective qualifies another, no comma is used, e.g.

A distinguished foreign author, a bright red tie.

 

2. To separate items (including the last) in a list of more than two items, e.g.

Potatoes, peas, and carrots; Potatoes, peas, or carrots; Potatoes, peas, etc.; Red, white, and blue.

• But A black and white TV set.

3. To separate co-ordinated main clauses, e.g.

Cars will turn here, and coaches will go straight on.

But not when they are closely linked, e.g.

Do as I tell you and you'll never regret it.

4. To mark the beginning and end of a parenthetical word or phrase, e.g.

I am sure, however, that it will not happen; Fred, who is bald, complained of the cold.

• Not with restrictive relative clauses, e.g. Men who are bald should wear hats.

5. After a participial or verbless clause, a salutation, or a vocative, e.g.

Having had breakfast, I went for a walk;
The sermon over, the congregation filed out or The sermon being over, (etc.);
My son, give me thy heart.

• Not The sermon, being over, (etc.)
• No comma with expressions like My friend Lord X or My son John.

6. To separate a phrase or subordinate clause from the main clause so as to avoid misunderstanding, e.g.

In the valley below, the villages looked very small;
He did not go to church, because he was playing Golf;
In 1982, 1918 seemed a long time ago.

• A comma should not be used to separate a phrasal subject from its predicate, or a verb from an object that is a clause:
  A car with such a high-powered engine, should not let you down and They believed, that nothing could go wrong are both incorrect.

7. Following words introducing direct speech, e.g. They answered, 'Here we are.'

8. Following Dear Sir, Dear John, etc., in letters, and after Yours sincerely, etc.

• No comma is needed between month and year in dates, e.g. In December 1982 or between number and road in addresses, e.g. 12 Acacia Avenue.